It may become necessary to detect the position of a metal object in a determined area, particularly in a plane area, for example, to detect a movement path of a metal object moving in a plane area or when metal objects are distributed in one area, to detect their distribution pattern. A specific example of the former is to detect a movement path of game play media in a game machine.
With some game machines, a player moves a metal substance, such as a metal ball, in a specific space set in the game machine and may or may not win the play depending on the destination of the metal ball. Pinball machines are typical of such game machines; with a pinball machine, a player plays a game by dropping a metal ball called a "pinball" in a space sandwiched between parallel planes in which a large number of obstacles are located.
A general pinball machine has a base board for providing a space required to move pinballs, a glass plate spaced from the base board at a given interval to cover the base board, and a propelling mechanism for propelling pinballs into the space provided by the base board and the glass plate. The pinball machine is set up so that the base board becomes substantially parallel to the vertical direction. The base board is formed with a plurality of safe holes for the player to enter a pinball in for a winning game play through which the pinball is discharged from the base board and an out hole into which pinballs that have not entered into the safe holes are finally collected for discharging the pinballs from the base board.
A large number of pins (nails) are set up substantially vertically to the base board in a state in which they project from the base board up to a distance which is as long as the diameter of a pinball, to act as obstacles with which pinballs dropping along the base board frequently collide, causing their motion direction to fluctuate. The pins are located on the base board with a distribution determined so as to guide pinballs colliding with the pins toward or away from the safe holes while causing the motion directions of the pinballs to fluctuate.
Incidentally, winning game play conditions at each pinball machine need to be managed at pinball parlors having a large number of such pinball machines. That is, it is necessary to sense whether or not a pinball enters each safe hole.
A conventional metal object detection system for such purposes is described in Japanese Patent Laid-Open No. Hei 4-122375.
In the gazette, a sensor comprising transmission lines and reception lines is proposed. That is, looped transmission lines are installed on one face of a wiring board and a plurality of parallel looped reception lines are installed on the opposing face of the wiring board crossing the transmission lines, so that the reception lines are electro-magnetically coupled with the transmission lines. The transmission lines and reception lines of this sensing matrix are connected to a transmission circuit and reception circuit of a controller, a signal current is made to flow into the transmission lines in sequence, and current induced by the signal current is taken out for each reception line in sequence, whereby the presence or absence of a metal object is detected from the induced current detected at the reception circuit and the position of the metal object can be detected from a combination of the transmission line on which the signal current flows and the reception lines on which the induced current is received. That is, the sensing matrix has the intersections of the transmission and reception lines as sensing units, which are placed like a matrix.
In such a metal object detection system, when a metal object exists at one point, it generates a response in the sensing unit nearest to its position and the sensing units surrounding it are also affected. Particularly, the effect is large if the transmission and reception lines are placed at a high density. Thus, if simply a signal output of a sensing unit is found and it is determined that a metal object exists at the position of the sensing unit, it is also judged that a metal object, such as a pinball, exists in the vicinity of the position where it is actually present. Thus, the position of the metal object cannot be detected accurately.
Particularly, to monitor whether or not a metal object, such as a pinball, enters one sensing unit, even if a pinball exists only in the vicinity of the sensing unit, the sensing unit also responds to it, outputting a signal as if a metal object actually existed. Thus, although a metal object does not actually enter the sensing unit, it is erroneously determined that a metal object enters the sensing unit.